1. Field of the Invention
The present invention relates to a method of fabricating a high aspect ratio, freestanding microstructure and more particularly a freestanding membrane containing an array of cylindrical cavities.
2. Description of the Prior Art
High aspect ratio microfabrication was known as the xe2x80x9cLIGAxe2x80x9d process, (Lithography, Plastic Forming, and Galvanoforming). In this method, an intense, highly collimated beam of X-rays from a synchrotron (5 to 15 keV in energy) is used to expose polymethyl methacrylate (PMMA), an X-ray sensitive resist that can be up to several millimeters in thickness. The exposed PMMA microstructure can be used as a stand-alone component or as a sacrificial mold for the fabrication of metallic components by electroforming.
After the exposure and development steps in the LIGA process, PMMA microstructures a few microns in width and several hundred microns high can be fabricated. The height of any given microstructure divided by its width is commonly known as the xe2x80x9caspect ratioxe2x80x9d and the LIGA process can achieve aspect ratios in excess of 50:1. Nevertheless, this process requires X-ray compatible masks and access to a synchrotron. Processes requiring only UV radiation and UV-transparent masks are much more economical. Interest has thus increased in a competitive process to LIGA that uses SU-8, a negative-acting photoresist that is sensitive to ultraviolet radiation, rather than X-rays. This resist is sufficiently transparent at the exposing wavelength (typically 365 nanometers) that thickness"" as great as one millimeter can be patterned by exposure tools commonly available to any semiconductor fabrication or microfabrication facility. Since the SU-8 process uses industry standard quartz masks, and requires no specialized equipment such as a synchrotron for exposure, it is much more economical than the LIGA process and is used to fabricate a wide variety of components with dimensions on the order of a few hundred microns. As with the LIGA process, microstructures fabricated from a negative photoresist 8 can be used as the final components in products such as watch gear assemblies, or as sacrificial molds for the mass-production of metal components using electroforming.
Components fabricated with negative photoresists have been shown with aspect ratios of 20:1. However, experiments have shown that these aspect ratios are not nearly as great for cylindrical cavity""s as they are for cylindrical pillars with identical diameters. This effect becomes particularly pronounced when the cavity diameter is less than 100 microns.
A previous patent describing the process for producing multi-layer microstructures using the SU-8 resin is found in European Patent EP 0,851,295 to Lorenz and Renaud. This patent describes a method involving use of a sacrificial coating on the support plate and use of at least two thick layers of SU-8 to form the final microcomponent.
U.S. Pat. No. 5,026,624 to Day et al. describes a composition for photoimaging which can be used as the negative photoresist. This patent describes photoimagable compositions, and particularly a cationically polymerizable epoxy resin system having photoinitiators and optionally photosensitizers added thereto which resin system has improved rheological properties and also improved photoimaging properties.
There is interest to develop microstructures fabricated with negative photoresists as essential components in X-ray detectors known as Gas Electron Multipliers, and electron photomultipliers known as microchannel plates.
An object of the present invention is to provide a method of fabricating high aspect ratio, freestanding three-dimensional microstructures. An additional object is to fabricate membranes that are composed of arrays of cylindrical cavity microstructures.
A method of fabricating a high aspect ratio, freestanding three-dimensional microstructure, comprises the application of a layer of negative photoresist sensitive to ultraviolet radiation with a thickness between 50 microns and 700 microns; whereby the photoresist, is formed by a lacquer containing a polyfunctional epoxy composition, a photoinitiator containing triarylsulfonium salts, and a solvent that is cast on a UV absorbent substrate. Then heating the layer between 90xc2x0 C. and 95xc2x0 C. for a period depending on the thickness deposited to cause the solvent to be evaporated. The UV illumination is carried out at a wavelength of about 365 nm and an intensity of 200 to 100 millijoules/cm2, depending on thickness of the layer, through a mask corresponding to the impression desired whereby the UV absorbent substrate filters out the UV radiation. The resist is baked for at least 5 minutes at about 95xc2x0 C. to ensure cross-linking in the UV-exposed regions. The unexposed photoresist is developed by means of a solvent chosen from among GBL (gamma butyrolactone) and PGMEA (propylene glycol methylethyl acetate). Lastly, the microstructure obtained by this procedure is separated from the UV absorbent substrate.